1. Field of the Invention
The present invention relates generally to an integrated circuit for biasing a quasi-linear bipolar device, and more specifically to a voltage controlled single-package integrated circuit device capable of thermal compensation for biasing such a device.
2. Description of the Prior Art
It is well-known in the art that biasing circuitry is required for quasi-linear devices such as bipolar RF transistors. Quasi-linear devices are characterized as being in the common-emitter configuration with a conduction angle of 180 degrees to 360 degrees. The biasing circuitry must supply the proper current levels required by the device while compensating for drifts in the device operating point brought about by changes in ambient temperature and operating conditions. Systems which are not protected from temperature fluctuations are more susceptible to ambient temperature changes and system heat-sinking properties which are not optimal. If operating point drift is not addressed and remedied, catastrophic device failure, or "thermal runaway" may result.
Because of this potential danger caused by thermal variations, biasing circuitry has long monitored and compensated for fluctuations in temperature. Typically, the base-emitter voltage level of the RF device being biased is monitored and, when increases in temperature cause the voltage level to drop, the voltage is stabilized by decreasing the amount of current in the RF circuit. However, biasing circuitry of the prior art has usually included multiple active components, capacitors, and inductors, on a PCB. Since these components are usually fabricated by different suppliers, the RF characteristics of the active devices typically have different geometries and, thus, often do not exactly match the characteristics of the RF device being biased. In addition, the use of discrete components utilizes more valuable board space than would a biasing circuit on a single integrated circuit. This is very important in today's market of high performance, low cost, small RF power hybrid modules.
Besides thermal tracking of the biased device, another important concern is the ability to change class of operation as system demands dictate. Typical quasi-linear modes of operation include classes A, AB1, AB2, and B. It may be desirable to change a system's mode of operation to any one of these. For example, if more efficiency and less linearity of operation is required, it may make sense to change class of operation from class AB1 to AB2. The class of operation of an RF device is typically altered by varying the voltage supplied to the device. However, the prior art has not successfully addressed this need with a single-package integrated circuit device.